Turbine blade construction



P 9 BQHAAS 3,148,954 nmsxus BLADE cons'mucnon Filed June 1 3. 1960.INVENTOR Bows HAAS\ DECEASED BY IRENE HAAS BY W 9 M ATTORNEYS UnitedStates Patent 3,148,954 TURBINE BLADE CONSTRUCTION Boris Haas, deceased,late of Gaiglstrasse 20/1, Munich, Germany, by Irene Haas, Renate Haas,Dieter Haas, and Detlev Haas, heirs, all of Munich, Germany Filed June13, 1960, Ser. No. 35,835 4 Claims. (Cl. 29-1961) The present inventionrelates to turbine guide and rotor blades made of a non-scaling metallicmaterial with high creep strength which are exposed during operationthereof to fluctuating temperature primarily exceeding 600 C.

The present invention aims at an improved resistance of the bladingmaterial against cracks caused by temperature changes and againstplastic deformations.

Cracks due to temperature changes arise in connection with turbineblades which are exposed frequently to abrupt heating and cooling,especially, however, to alternate rapid heating and cooling thereof.Such conditions occur, for example, with the guide and rotor blades ofmovable gas turbines which have to be started and stopped frequently, orwhich have to be accelerated and decelerated over very short periods oftime.

Temperature-change cracks are caused by stresses which are caused by theuneven temperature distribution over the cross-section of the turbineblade. With a blade heated throughout, the outer zone thereof,especially during the initial period of heating thereof, has a highertemperature than the core thereof. The temperature difference reachesits maximum after a relatively short time. However, a completetemperature equalization takes place only very rarely so that almostalways a continued residual temperature gradient from the surface towardthe core of the blade remains in effect.

The outer zone which seeks to expand to a greater extent by reason ofits higher temperature than the core zone is prevented from doing so bythe molecular coherence of the material. The blade, instead, assumes amean expansion over. the entire cross-section thereof wherebycompression stresses are produced within the rim zones and tensionalstresses within the core zone thereof. With a universal cooling of theblade body, the respective signs of the stresses change, i.e., thecompression stresses of the outer zone now become tensional stresses,and the tensional stresses of the core zone now become compressionstresses.

Especially with movable gas turbine in which sudden changes in load areunavoidable, the material of the turbine blading is endangered to a veryhigh degree. By reason for the extraordinarily rapid heating duringacceleration and of the abrupt cooling during deceleration of theturbine unit as well as by reason of the relatively poor thermalconductivity of the high-temperature resistant material for which onlyeither high-grade austenitic steels or almost completely iron-freenickel and cobalt base alloys are normally considered, a relativelylarge temperature difference results between the core and surface of theblade, and a relatively large temperature gradient exists near thesurface of the blade. Since the core by reason of its largecross-section fails to undergo, in practice, essentially any elasticdeformation, the material disposed near the surface is forced to absorbpractically the entire difference in expansion. As a result thereof,very high stresses are produced which almost always exceed to a largeextent the elastic limit and lead to plastic deformations in theoutermost layer of the material. Upon exceeding the elastic limit, theretakes place a flow of the outer fibers of the material whereby theplastic deformation is the greater the higher the temperature stress.The amount of plastic deformation has to be 3,148,954 Patented Sept.15,, 1964 ICC redeformed during cooling of the blade. Consequently, theouter fiber is plastically deformed back and forth until the separatingstrength of the material has been lowered to such an extent that it isexceeded during cooling by the tensional stresses and the incipientcrack results therefrom.

The recognition that the temperature-change cracks are in effectendurance failures or fatigue fractures in the prolonged strength regionand that the cracks occur the sooner the greater the alternate plasticdeformations, leads to the inventive concept of the present applicationto reduce the magnitude thereof with the aid of a coating or layer.

Consequently, the resistance of the blade material is to be increased inaccordance with the present invention or an extension of the operatingperiod up to the point of the occurrence of the aforementioned damagesis to be attained in the alternative.

For that purpose, the present invention proposes to provide the inletand outlet edge of the blade with an auxiliary metallic protective coveror coating adhering rigidly to the core material whereby the thicknessof the metallic auxiliary coating amounts to between one percent andtwelve percent of the blade cross-sectional length, the cocflicient ofthermal expansion of which, within the temperature range above 600 C.,is smaller than 0.8 times the coefiicient of expansion of the corematerial and the alternate plastic deformability of which is at leastequal to that of the core material while the non-scaling characteristicsthereof are also approximately equal to that of the core material.

As protective materials for the alloys are particularly suitable, forexample, the known so-called non-scaling ferrite-austenitic or pureferrite steels, such as, for example, those having approximately 25%chromium, 4% nickel and other additives or those with approximately 13%to 30% chromium and other additives or ductile chrome. These alloys havecoefficients of expansion which have the desired and necessaryrelationship, as mentioned hereinabove, with respect to the alloys to beprotected thereby, and which also have sufiicient plastic deformability.The non-sealing characteristics thereof are equal or better than thoseof the nickelor cobaltcontaining materials whereas the resistanceagainst reducing and sulphur containing gases and against the vanadiumpentoxide attacks is even better. Since the corrosion attacktakes placepredominately along the thin edges of the blade, this property may bevery valuable. It is true that the fatigue strength of theaforementioned protective materials is relatively slight, however, theload capacity or bearing strength of the protected blade cross-sectionis reduced thereby only slightly as com:

The formation of temperature-change cracks is effectively aided usuallyby corrosion of the material surface as a result of theheat-transferring medium in contact therewith. Most of the time, itoccurs as grain-boundary oxidation and elfects, in addition to localstress increases as a result of the notch elfect, a strong decrease ofthe separating resistance, especially, if a longer operating period withrelatively high temperatures lies between heating and cooling.Consequently, the protective surface layer, according to the presentinvention, is to have, as a further characteristic, a greater corrosionand nonscalingresistance than the core material. Possibly, an additionalcorrosion-resistant or non-scaling protective layer may be appliedadditionally over the layer with ing the corrosion resistance of theblade.

Protective surface layers in turbine blades are known per se. Protectivelayers, for example, made of oxidic, mineral, glass-like or ceramicmaterials have already been used heretofore in the prior art. However,these prior art protective layers had primarily the purpose to protectthe core material against chemical and thermal influences. Additionally,metallic coatings are known in the prior art in connection with thesteam turbine blades made of unalloyed or relatively low-alloy steel.For that purpose, a protective layer had been proposed in the prior artwhich contained nickel or cobalt or which consisted of a nickel steel.If a low-alloy steel alloyed with nickel or also Possibly pure nickel isused for achieving the protective effect, then such protective layeralways has the same co efficient of expansion as the core material.Austenitic iron and nickel alloys with to 27% nickel thereby offer thebest resistance. However, in particular the latter alloys have acoefficient of expansion that is approximately 50% larger than that ofthe non-alloyed or low-alloy steels. With cobalt, the conditions areessentially similar.

Consequently, whereas the prior art application of protective surfacecoatings served the purpose of protection against corrosion, accordingto the present invention, the stress peaks and/or plastic deformationswhich occur as a result of prevented deformation in the surface arereduced with turbine blades made of a non-scaling, metallic alloy with ahigh creep strength by the use of an adhering coating having a smallercoeflicient of expansion. For example, with an operating temperature of800 C. and a temperature difference of 400 C., a decrease of thetemperature stresses by about both during heating as well as duringcooling is mathematically obtainable if a blade made of austenitic steelhaving a coefficient of expansion of l8 l() l./ C. is coated or coveredwith a corresponding layer of an alloy having a coefficient of expansionof 14 10 l/ C.

By reason of the decrease in the temperature stresses, the amount ofplastic alternate deformation, the so-called alternate sliding, iscorrespondingly decreased in the rim or edge fibers of the blade andtherewith the life-length of the turbine blade is considerablyincreased. The coefficient of thermal expansion of the protective layercan be adjusted to the required value by any known suitable change inthe chemical composition thereof.

If the protective surface layer at the same time has a better plasticdeformability than the core material, then the protective layer iscapable of absorbing more frequently the amount of plastic deformation,i.e., the alternate sliding.

By reason of the increased corrosion resistance of the coating material,the unfavorable influence of the corrosion of the contacting medium onthe life-length of the particular part is thereby effectivelyeliminated.

The surface layer according to the present invention may be formedeither by coating or by diffusion. The coating may be made, for example,by dipping or immersion in a fused mass, by casting it around the blade,by build-up welding, by plating with rollers or by extrusion presses, byvaporization, by spraying or by galvanic deposition.

Accordingly, it is an object of the present invention to effectivelyeliminate the disadvantages and shortcomings of the turbine bladeconstructions used heretofore in the prior art.

Another object of the present invention resides in the provision of aprotective layer for a turbine blade which effectively prevents theformation of cracks along the edges thereof normally caused by largechanges in the operating temperatures.

Still another object of the present invention resides in the provisionof a protective layer for a turbine blade which makes possible improveduse of the blade in connection with gas turbine engines exposed tofrequent accelerations and decelerations.

A further object of the present invention resides in the provision of aprotective layer having such a coeflicient of expansion as to minimizepeak stresses throughout the blade cross-section while at the same timeincreasing the corrosion resistance of the blade.

These and other objects, features and advantages of the presentinvention will become more obvious from the following description whentaken in connection with the accompanying drawing which shows, forpurposes of illustration only, one embodiment in accordance with thepresent invention, and wherein FIGURE 1 is a cross-sectional view of ablade in accordance with the present invention,

FIGURE 2 is a diagram showing the temperature distribution along theblade of FIGURE 1, and

FIGURE 3 is a diagram showing the stress distribution within the bladeillustrated in FIGURE 1.

Referring now to the drawing, FIGURE 1 illustrates a blade cross-sectionof a turbine blade constructed in accordance with the present invention,the core 1 of which consists of a non-scaling metallic material withhigh creep strength and the inlet and outlet edges of which areprotected by layers or coatings 2. The metallic materials for theprotective layers in accordance with the present invention has acoefiicient of expansion within the temperature range above 600 C. whichis smaller than 0.8 times the coefficient of expansion of the corematerial while the resistance to alternate deformations is at leastequal to that of the core material whereas its non-scaling property isapproximately equal to that of the core material.

During acceleration of the turbine, a very pronounced differingtemperature distribution occurs in the longitudinal direction of theblade profile as shown by curve 3 of FIGURE 2. If the blade body is madeof only one and the same material, then this temperature distributionproduces a stress distribution illustrated in FIGURE 3 by full linecurve 4.

The relatively high peak stresses along the edges of the blade are quitenoticeable in FIGURE 3.

The blade in accordance with the present invention, the edges of whichare provided with a protective layer of thickness 5, exhibits with thesame temperature distribution as illustrated in FIGURE 2, a stressdistribution illushated by the dash curve 6 in FIGURE 3 from which it isclearly noticeable that a decrease in the stresses along the edgesresults therefrom.

With relatively rapid cooling, the same curves will be obtained for thetemperature and stress distributions, however, of mirror image-likeconfiguration with respect to the X-axis. The X-axis of the temperaturediagram of FIGURE 2 corresponds in that case to the operatingtemperature instead of to the initial temperature While the rim and corestresses change the signs thereof.

It is clearly visible from the curve 6 of FIGURE 3 that that thicknessof the protective layer 2 has to be so matched and selected that thestress at the transition between protective and core material is notsignificantly greater than the reduced rim stress. This effect isobtained by determining the thickness of the protective material 2 insuch a manner that, depending on the core material and on the protectivematerial and on the existing temperature gradient, it lies between 1%and 12% of that cross-sectional dimension in the direction of which thelargest temperature differences occur. Consequently, in the illustratedexample, the thickness 5 has to amount to about 0.01 to 0.12 times thewidth 7 of the blade.

While I have shown and described one embodiment in accordance with thepresent invention, it is understood that the same is not limited theretobut is susceptible of many changes and modifications within the spiritand scope of the present invention, and I, therefore, do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are encompassed by the scope ofthe appended claims.

What is claimed is:

1. A turbine blade for use at temperatures above 600 C. comprising acore essentially consisting of austenitic steel having a coefiicient ofexpansion of about 18 X l./ C., said blade being provided at the leadingand trailing edges with a protective layer of metallic material adheringto the austenitic steel of said core body, said metallic material ofsaid protective layer comprising a steel alloy selected from the groupconsisting of chromium nickel steel of approximately 25% and 4% nickeland chromium steels having 1330% chromium, said alloy having acoeflicient of expansion of about 14 10- l./ C., said protective layerhaving a thickness of between 1 to 12% of the cross-sectional length ofthe blade taken in the direction of maximum difference of temperature insaid blade, the alternate plastic deformability of said protective layerbeing at least equal to that of said core.

2. A turbine blade for use at temperatures about 600 C., comprising acore essentially consisting of a metallic material with a high creepstrength selected from the group consisting of high-grade austeniticsteels and almost completely iron-free nickel and cobalt base alloys andprovided at the leading and trailing edge thereof with a metallioprotective layer adhering to the blade core material and selected fromthe group consisting of chromium nickel steel of approximately 25% and4% nickel and chromium steels having 13-30% chromium, the thickness ofsaid protective layer amounting to between 1 to 12% of thecross-sectional length of the blade taken in the direction of maximumdifierence of temperature in said blade, and the coefiicient of thermalexpansion of said metallic protective layer Within the temperature rangeabove 600 C. being smaller than approximately 0.8 times the coefiicientof expansion of the core material, and the alternate deformability ofsaid protective layer being at least equal to that of the core material.

3. A turbine blade according to claim 2, wherein said metallicprotective layer has a greater resistance to scaling than said bladecore material.

4. A turbine blade according to claim 2, wherein said metallicprotective layer has a greater corrosion resistance than said blade corematerial.

References Cited in the file of this patent UNITED STATES PATENTS2,034,278 Becket et a1 Mar. 17, 1936 2,447,896 Clarke Aug. 24, 19482,497,151 Clark et al Feb, 14, 1950 2,586,100 Schultz Feb. 19, 19522,606,741 Howard Aug. 12, 1952 2,763,919 Kempe et al Sept. 25, 19562,861,327 Bechtold Nov. 25, 1958 2,946,681 Probst et al July 26, 1960FOREIGN PATENTS 309,235 Great Britain Apr. 11, 1929

1. A TURBINE BLADE FOR USE AT TEMPERATURES ABOVE 600* C. COMPRISING ACORE ESSENTIALLY CONSISTING OF AUSTENITIC STEEL HAVING A COEFFICIENT OFEXPANSION OF ABOUT 18X10**-6 1./C., SAID BLADE BEING PROVIDED AT THELEADING AND TRAILING EDGES WITH A PROTECTIVE LAYER OF METALLIC MATERIALADHERING TO THE AUSTENITIC STEEL OF SAID CORE BODY, SAID METALLICMATERIAL OF SAID PROTECTIVE LAYER COMPRISING A STEEL ALLOY SELECTED FROMTHE GROUP CONSISTING OF CHROMIUM NICKEL STEEL OF APPROXIMATELY 25% AND4% NICKEL AND CHROMIUM STEELS HAVING 13-30% CHROMIUM, SAID ALLOY HAVINGA COEFFICIENT OF EXPANSION OF ABOUT 14X10**-6 1./ *C., SAID PROTECTIVELAYER HAVING A THICKNESS OF BETWEEN 1 TO 12% OF THE CROSS-SECTIONALLENGTH OF THE BLADE TAKEN IN THE DIRECTION OF MAXIMUM DIFFERENCE OFTEMPERATURE IN SAID BLADE, THE ALTERNATE PLASTIC DEFORMABILITY OF SAIDPROTECTIVE LAYER BEING AT EQUAL TO THAT OF SAID CORE.